1. Field of the Invention
This invention relates to the field of document processing systems, such as copying machines, facsimile machines or Optical Character Recognition ("OCR") systems. The invention particularly relates to a method and apparatus for detecting a skew angle of a document image.
2. Description of the Related Art
Bit-mapped images of a document handled by a document processing system, such as a digital copying machine, a facsimile machine, or an image scanner, are becoming more popular in the office. In these systems, skew or tilt of the document image produces undesirable results. Furthermore, skew or tilt of a document reduces a recognition accuracy of an OCR system.
To remove document skew, it is first required to accurately detect the skew of the document image. A paper presented by M. Hase and Y. Hoshino entitled "Periodic Characteristics in Printed Documents" published in the Transactions of Japanese Institution of Electronics and Communication Engineer, '82/2 Vol J65-D No. 2, provides a method which utilizes a two-dimensional Fourier Transform. In this method, character lines of a document image were determined. When the character lines are equally spaced and aligned in parallel, a result of the two-dimensional Fourier Transform of the document image data showed a maximum value which is perpendicular to the direction of the character lines. However, this method is not very sensitive at estimating the skew angle of the document and comes with high computational cost.
Another method was presented by Japanese Laid-Open Patent Application Publication 2-116987 (1990). This publication discloses the detection of skew angle utilizing the Hough Transform after detection of a character and character lines from an input image. However, the above-mentioned technology has been made on the premise of the practice of detection of a character and character lines in input images. Therefore, this technique is not able to detect skew of a document which has no character lines. Further, the Hough Transform requires extensive computation time, and is found not to be sufficiently sensitive enough to the skew angle.